Flame spraying involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface and bond thereto. A conventional flame spray gun is used for the purpose of both heating and propelling the particles. In one type of flame spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., below 100 mesh U.S. standard screen size to about 5 microns.
In typical plasma flame spraying systems for spraying powder, an electric arc is created between a water cooled nozzle (anode) and a centrally located cathode. An inert gas passes through the electric arc and is excited thereby to temperatures of up to 30,000.degree. F. The plasma of at least partially ionized gas issuing from the nozzle resembles an open oxy-acetylene flame. A typical plasma flame spray gun is described in U.S. Pat. No. 3,145,287.
The electric arc of such plasma spray guns, being as intense as it is, causes nozzle deterioration and ultimate failure. One cause for such deterioration is the fact that the arc itself strikes the nozzle/anode at a point, thereby causing instantaneous local melting and vaporizing of the nozzle surface. Deterioration is also caused by overheating the nozzle to the melting point so that part of the nozzle material flows to another location which may eventually cause the nozzle to become plugged.
There are varying degrees and rates associated with each cause for nozzle deterioration. Experience has shown that wall erosion, ultimately causing the coolant to burst through the nozzle wall, is another cause of nozzle failure. When the jacket bursts, coolant water is released into the arc region, resulting in a locally intense electric arc, causing parts to melt. Once a meltdown has occurred, gun repair can be very costly. The nozzle deterioration and failure problem is particularly severe at high power levels.
In seeking to overcome this problem, plasma flame spray guns have been designed with easily changed water cooled nozzles. During operation, water coolant is forced through passages in the nozzle to cool the nozzle walls. Even so, gradual, or sometimes rapid, deterioration occurs and, as a precaution against failure, the nozzles are usually replaced after a given number of hours of service. This practice of replacing the nozzle periodically, however, is quite costly because the interchangeable nozzles are fairly expensive and many nozzles with considerable life remaining are thereby discarded.
U.S. Pat. No. 4,430,546 describes a plasma spray gun nozzle with a thin wall and an annular coolant passage to provide extended life. Specific dimensions of the wall and passage are disclosed to assure maximum nozzle life. That development substantially advanced the life expectancy of nozzles, especially in heavy duty plasma guns. However, the construction of the nozzle incorporating the coolant passage, as taught therein, is not conducive to achieving low cost for parts, particularly with respect to nozzle replacement. In particular a one-piece unitary nozzle containing cooling passages is expensive. An alternative method suggested in the above-named patent is a part of "clam shell" parts that fit about the nozzle, but these are not easy to use and can allow leaking of the coolant.
Another form of nozzle insert in an arc torch device containing an annular cooling passage is shown in U.S. Pat. Ser. No. 3,106,633. However, before the nozzle can be removed replaced, two other components must be removed including the part providing the outer wall of the annular passage, which must be threaded out of the arc torch device.
Therefore, it is an objective of the present invention to provide for a plasma spray gun an improved nozzle assembly containing a coolant passage.
It is a further object to provide a novel nozzle assembly which contains a coolant passage for extended nozzle life in a plasma spray gun and which allows convenient and low cost replacement of the nozzle.
It is yet a further object to provide an improved plasma spray gun including a nozzle assembly which contains a coolant passage and allows convenient and low cost replacement of the nozzle.
It is another object to provide a plasma spray gun including a nozzle assembly with a coolant passage and having improved operation and low cost maintenance.